Re: [PATCH 1/9] KVM: SVM: Never reject emulation due to SMAP errata for !SEV guests

[Liam Merwick]

On 20/01/2022 01:07, Sean Christopherson wrote:
> Always signal that emulation is possible for !SEV guests regardless of
> whether or not the CPU provided a valid instruction byte stream.  KVM can
> read all guest state (memory and registers) for !SEV guests, i.e. can
> fetch the code stream from memory even if the CPU failed to do so because
> of the SMAP errata.
>
> Fixes: 05d5a4863525 ("KVM: SVM: Workaround errata#1096 (insn_len maybe zero on SMAP violation)")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Liam Merwick <liam.merwick@xxxxxxxxxx>

> ---
>   arch/x86/kvm/svm/svm.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 6d31d357a83b..aa1649b8cd8f 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4257,8 +4257,13 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int i
>   	bool smep, smap, is_user;
>   	unsigned long cr4;
>   
> +	/* Emulation is always possible when KVM has access to all guest state. */
> +	if (!sev_guest(vcpu->kvm))
> +		return true;
> +
>   	/*
> -	 * When the guest is an SEV-ES guest, emulation is not possible.
> +	 * Emulation is impossible for SEV-ES guests as KVM doesn't have access
> +	 * to guest register state.
>   	 */
>   	if (sev_es_guest(vcpu->kvm))
>   		return false;
> @@ -4318,9 +4323,6 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int i
>   	smap = cr4 & X86_CR4_SMAP;
>   	is_user = svm_get_cpl(vcpu) == 3;
>   	if (smap && (!smep || is_user)) {
> -		if (!sev_guest(vcpu->kvm))
> -			return true;
> -
>   		pr_err_ratelimited("KVM: SEV Guest triggered AMD Erratum 1096\n");
>   		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>   	}